✨ Add skill projects modal with Headless UI (Story 2.5)

- Add GET /skills/{slug}/projects endpoint with level progression
- Install @headlessui/vue for accessible modal
- Create SkillProjectsModal with Dialog component:
  - Focus trap and keyboard navigation (automatic)
  - Fade + scale transitions with backdrop blur
  - prefers-reduced-motion support
- Create ProjectListItem with thumbnail and level display
- Integrate modal in competences.vue page
- Add translations for related projects UI

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

api/routes/api.php

@@ -11,3 +11,4 @@ Route::get('/health', function () {
 Route::get('/projects', [ProjectController::class, 'index']);
 Route::get('/projects/{slug}', [ProjectController::class, 'show']);
 Route::get('/skills', [SkillController::class, 'index']);
+Route::get('/skills/{slug}/projects', [SkillController::class, 'projects']);